A godet for heating an advancing yarn is disclosed in German Patent DE-A 33 16 744. In the godet, the frequency of the primary current is variably controlled as a function of the measured temperature so that the temperature can be kept constant. In maintaining the temperature constant by controlling the frequency, however, it is unavoidable that at a lower energy demand, the frequency lies in electrically and/or magnetically unfavorable ranges. The advantage of the good controllability is therefore obtained at the expense of impaired efficiency and an increase in the power loss to be dissipated.
The godet disclosed in German Application DE-OS 16 60 215 is operated at a constant primary frequency- To control the temperature, the primary circuit is connected and disconnected in time intervals dependent on the measured temperature. This godet does not exhibit the aforesaid decrease in efficiency. However, a secondary winding is necessary which is highly conductive and which consists of a copper or brass ring which is securely connected to the inner circumference of the magnetic iron casing of the godet. Such a secondary winding, which must be firmly and securely connected to the godet casing under all types of thermal and mechanical stresses, is difficult and expensive to manufacture.
German Patent 18 04 777 discloses a godet which is additionally provided with a hollow casing filled, in part, with a fluid and in addition with the vapor of this fluid. This godet achieves good temperature uniformity over the entire length of the godet. The disadvantage of this approach lies again in its expense and complexity.